Some superconducting solenoids previously used in connection with NMR spectrometers have employed superconducting wire having a circular cross-section. In one configuration, the wire has been of monofilament form including a core formed of a superconducting material, such as Niobium-titanium alloy, surrounded by a sheet of non-superconducting metal having a relatively low room temperature conductivity, such as copper. Typically, the diameter of the circular cross-section wire has been between 0.016" to 0.030". The superconducting, monofilament wire is typically wound as a helix having multiple, concentric layers, between each of which is located a thin dielectric sheet, such as Mylar. Adjacent layers usually have to be wound in opposite longitudinal directions, i.e., have oppositely directed pitches.
After the first layer of the solenoid has been wound, the subsequent layers are not true helices because the round wire tends to fall into the grooves between windings of the previously wound layer. Because adjacent layers are wound in opposite directions, each turn of a subsequently wound layer follows the pitch of the previously wound layer and then jumps over a turn of the immediately previously wound layer. Hence, there is a non-uniformity in the pitch of the winding between successive turns of every helical layer, except the first layer. This winding non-uniformity causes substantial radial and spiral gradients in the magnetic field which is produced when the superconducting solenoid is energized with a current, whereby the magnetic field is not homogeneous in the interior of the solenoid, where a tested sample, as well as the rf excitation and pick-up coils, are located. The spiral field gradient occurs because of the tendency of the round wire to follow the groove between adjacent wires of the previously wound layer. The jump over between adjacent turns of the same layer causes a radial field gradient to be produced. Previous attempts to correct these field gradients have been very difficult and expensive, and, where adopted have not proven successful.